482 A. TT. MASTERMAN. 
everywhere by the circum-oral band carrying long cilia, but 
is not itself ciliated except in one particular part (see below). 
The general arrangement of the area is a combination of 
bilateral (or planar) and axial symmetry. ‘The bilateral 
symmetry is perfect. ‘The band runs away from the region ; 
of the mouth in acomplicated loop which is identical on either 
side of the body. ‘There can usually be distinguished an 
antero-ventral, an antero-dorsal, and a postero-dorsal band. 
The original part of the band leading away from the mouth 
forms a postero-ventral fold joining with its fellow across the 
mouth. 
A view of the larva from the anterior end shows that the 
four anterior folds are arranged symmetrically round the 
central axis and tend towards an axial symmetry, which 
becomes more pronounced in the most complex larvee (cf. 
Morgan’s “ Bimini larva”). 
The phyletic history of this band is deals a gradual 
extension of a bilateral loop, conforming by degrees to the ~ 
axial symmetry of the larva. It does not appear, in the 
North Sea larva, to form loops or tentacles in its course. 
The posterior band remains as a simple circular band in all 
known Tornarie, and traverses the posterior part of the 
extra-oral area. 
An examination of the living larva shows that this extra- 
oral band, like the peri-anal band of Actinotrocha, is purely 
motor in function. Only when its cilia are in action does the 
larva move with any activity. The cilia are curved, and all 
strike the water backwards, with a resultant effect along the 
antero-posterior axis of the larva. Its purely motor function 
explains the limitations of the band at all stages to a simple 
circle symmetrical with the long axis. 
On the other hand, the circum-oral’ band does not 
appear to any great extent to subserve locomotion. On 
occasions when the extra-oral band is not in motion the 
circum-oral band may be very active with but little or no 
effect on the general movements of the larva. The main 
function of the circum-oral band appears to be that-of food- 
